Flame safety system for insitu process analyzer

ABSTRACT

A method of operating a process a combustion analyzer having a measurement cell is provided. The method includes exposing the measurement cell to exhaust of a combustion process where fuel and oxygen are combined in a burner to produce a flame. The measurement cell is heated to a temperature above a flashpoint of the fuel. When a condition is detected, such as a fault or abnormal situation, gas is directed to the measurement cell to form a gaseous barrier between the measurement cell and unburned fuel while the detected condition exists. Once the condition abates, the gas flow is disengaged and process combustion gas measurements are provided

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of and claims priority of U.S.patent application Ser. No. 12/503,275, filed Jul. 15, 2009, the contentof which is hereby incorporated by reference in its entirety.

BACKGROUND

Industrial process industries primarily rely upon energy sources thatinclude one or more combustion processes. Such combustion processesinclude operation of a furnace or boiler to generate energy fromcombustion, which is then used for the process. While combustionprovides relatively low-cost energy, its use is typically regulated andcombustion efficiency is sought to be maximized. Accordingly, one goalof the process management industry is to reduce the production ofgreenhouse gases by maximizing combustion efficiency of existingfurnaces and boilers.

In situ or in-process analyzers are commonly used for the monitoring,optimization, and control of combustion processes. Typically, theseanalyzers employ sensors that are heated to relatively high temperaturesand are operated directly above, or near, the furnace or boilercombustion zone. Known analyzers, such as that sold under the tradedesignation X-Stream O₂ Combustion Flue Gas Transmitter available fromRosemount Analytical Inc. of Solon, Ohio (an Emerson Process Managementcompany), often employ zirconia oxide sensors heated to a temperatureabove approximately 700° Celsius (1300° Fahrenheit). If the combustionprocess should suffer a flame out condition, raw fuel and air are couldbe exposed to this sensor which, by virtue of its elevated temperature,could become an ignition source with the possibility of precipitating anexplosion.

Known analyzers generally employ a sintered metal or other diffuserpositioned between a measurement cell and the process combustion gas toallow the process gas to diffuse to the measurement zone whileminimizing flow effects and reducing measurement cell contamination. Thediffuser readily allows the process gas to contact the heatedmeasurement cell itself and, in the case of the process combustion gasis replaced by a flammable gas, enables an explosion. This situation canoccur if the combustion flame is extinguished and fuel continues toflow.

Some process analyzers are approved for hazardous area operation. Someapprovals include those provided by the Canadian Standards Association(CSA), Factory Mutual (FM), ATmosphares EXplosibles (ATEX), et cetera.Typically, hazardous area-approved analyzers include a flame arrestorthat is added over the diffuser with the intent of quenching, orotherwise inhibiting, an explosion that might occur in front of theheated measurement cell, thereby preventing the ignition of the largerfuel volume in the boiler or combustion zone. These flame arrestors havebeen tested and approved in the past. However, it is believed that thesafety provided by such arrestors can be improved. Moreover, theutilization of the flame arrestors may inhibit, to some degree, accessto the measurement cell thereby increasing measurement lag.

State of the art process safety systems generally provide a flamescanner to alert an operator and/or send an electrical signal indicatingthat the flame is extinguished and that raw fuel may be flowing. Fullyautomated systems immediately shut down fuel flow, while manual systemsgenerally require operator intervention.

A potentially hazardous situation can also arise during the initiallighting of the process burner or boiler, where fuel is introduced andan ignition source is used to initiate a flame. In some situations, rawfuel may reach the oxygen sensor (heated by its own heater to atemperature of 700° Celsius) which may provide a source of ignitionprior to the intended ignition source. This can cause a potential flashor explosion. Typically, either a flame arrestor is used on the oxygensensor or the analyzer is not powered during boiler or furnace startup.The non-powered analyzer is completely safe since the oxygen sensor isnot heated and thus cannot form an unintended source of ignition.However, since the analyzer is non-functional for 30-45 minutes afterstartup, the analyzer is unavailable during the critical combustionstartup phase. This can waste fuel and allow excessive emissions andinefficiencies. Thus, it is desired to have an analyzer system thatprovides both safe startup and fault condition operation while remainingreadily available.

SUMMARY

A method of operating a process combustion analyzer having a measurementcell is provided. The method includes exposing the measurement cell toexhaust of a combustion process where fuel and oxygen are combined in aburner to produce a flame. The measurement cell is heated to atemperature above the flashpoint of the fuel. When an exception or afault condition is detected, gas is directed to the measurement cell toform a gaseous barrier between the measurement cell and unburned fuelwhile the condition exists. Once the condition abates, the gas flow isdisengaged and process combustion gas measurements are provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of an in situ combustion process analyzerwith which embodiments of the present invention are particularly useful.

FIG. 2 is a diagrammatic exploded view of a process analytic oxygensensor useful in accordance with embodiments of the present invention.

FIG. 3 is a diagrammatic view of an in situ combustion process analyzerin accordance with an embodiment of the present invention.

FIG. 4 a is a diagrammatic view of a portion of a probe assemblyoperating during a normal condition.

FIG. 4 b is a diagrammatic view of a portion of a probe assembly duringcalibration.

FIG. 5 is a flow diagram of a method of operating an in situ combustionprocess analyzer in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Embodiments of the present invention generally provide a gaseous barrierbetween a heated process analytic sensor and a potentially flammable ordeleterious environment. Embodiments of the present invention willgenerally be described with respect to an in situ process analyticoxygen analyzer, but embodiments of the present invention are applicableto any process analytic sensor that operates at a temperature that canpotentially generate an unintended flashpoint for flammable or explosivematerials. Advantageously, embodiments of the present invention mayallow legacy process analytic hardware to operate in a new manner thatreduces or minimizes the potential for unintended ignition while alsoproviding the benefits of substantially immediate process analyticmeasurements once a combustion process is initiated.

FIG. 1 is a diagrammatic view of an in situ process combustion analyzerin accordance with the prior art. Transmitter 10 can be any suitableanalyzer including the X-Stream O₂ Combustion Flue Gas Transmitterlisted above. Transmitter 10 includes a probe assembly 12 that isdisposed within a stack or flue 14 and measures at least one parameterrelated to combustion occurring at burner 16. Typically, transmitter 10is an oxygen transmitter, but can be any device that measures anysuitable parameter related to the combustion process. Burner 16 isoperably coupled to a source of air or oxygen 18 and a source 20 ofcombustible fuel. Each of sources 18 and 20 is preferably coupled toburner through a valve of some sort to deliver a controlled amount ofoxygen and/or fuel to burner 16 in order to control the combustionprocess. Transmitter 10 measures the amount of oxygen in the combustionexhaust flow and provides an indication of the oxygen level tocombustion controller 22. Controller 22 controls one or both of valves24, 26 to provide closed-loop combustion control. Transmitter 10includes an oxygen sensor that typically employs a zirconia oxide sensorsubstrate to provide an electrical signal indicative of oxygenconcentration, content or percentage in the exhaust. Zirconia oxidesensors operate at a temperature of about 700° Celsius and thustransmitter 10 includes, within probe assembly 12, an electrical heaterthat is operably coupled to AC power source 29. AC power source 29 canbe a 110 or 220 VAC source that provides electrical energy to one ormore electrical heating elements within probe assembly 12 to heat thezirconia oxide sensor substrate to a suitable temperature.

As can be appreciated, should burner 16 experience a flameout condition,it is possible that raw fuel and air could continue to flow from sources20, 18, respectively, which materials could contact the hot zirconiaoxide sensor, which could provide an unintended source of ignition. Inorder to address flameout conditions, prior art methods (including thatillustrated in FIG. 1) generally include a flame scanner 28 disposed toprovide a signal indicative of the presence of flame 30 at burner 16.This flame scanner signal has been provided allow suitable reaction tothe flameout condition. In the past, the flame scanner signal has beenused to close a fuel valve and/or remove power from the analyzer therebyde-energizing the heater within probe assembly 12. In many cases, thisremoval of power allows rapid cooling of the zirconia oxide sensor to atemperature that is below the fuel ignition temperature, therebycreating a safe condition. However, upon restoration of flame 30 atburner 16, the analyzer heater power is restored but transmitter 10 isunavailable to provide combustion oxygen information until operatingtemperature of the zirconia oxide sensor is reached. This lag to reachoperating temperature is typically on the order of 10-45 minutes duringwhich the critical combustion startup phase may be wasting fuel and/orallowing excessive emissions and inefficiency.

FIG. 2 is a diagrammatic view of an in situ process combustion analyzerwith which embodiments of the present invention are particularly useful.Probe assembly 12 is generally configured to house sensor core assembly30 which includes diffuser 32 disposed proximate metal retainer 34,which is disposed next to measurement cell 36. As described above,measurement cell 36 is operable at an elevated temperature and theelevated temperature is provided by electrical heater 38. Measurementcell 36 and heater 38 are electrically coupled to transmitter 10 viaboard 40. Board 40 is configured to engage electronics board 42 inhousing 44. Board 40 also includes a plurality of gas inlets 46 and 48to receive reference air and calibration gas, respectively.

Zirconia oxide sensing technology has historically measured processoxygen by using ambient or instrument air as a reference (20.95%oxygen). Periodically, the sensor may need to be calibrated where aprecisely controlled amount of oxygen can be introduced to the sensorand exposed to measurement cell 36. Accordingly, ports 46 and 48 arecoupled to conduits that direct the reference and calibration gases tocell 36. The reference gas is provided to a side of the zirconia oxidesubstrate that is away from the process gas. During calibration,however, calibration gas is supplied to the side of the zirconiasubstrate that is opposed to the side exposed to reference gas. In thismanner, each side is exposed to a gas.

Embodiments of the present invention generally employ anynon-combustible gas, typically reference air or calibration gas, as apurge gas during a flameout condition to provide a continuously flowinggas barrier between measurement cell 36 and diffuser 32. In this manner,combustible, explosive, or otherwise deleterious materials that may beaccumulating within the flue are kept safely away from the hot (atemperature that is at or above the flashpoint of the combustible orexplosive material) surfaces.

FIG. 3 is a diagrammatic view of an in situ combustion process analyzerin accordance with an embodiment of the present invention. Transmitter50 includes probe assembly 52 containing therein a process gas sensorthat operates at a temperature that is high enough to ignite unburnedfuel from source 20 in the presence of air or oxygen from source 18 ifflame 30 is lost. As shown in FIG. 3, the signal from flame scanner 28,or some other robust digital input, is coupled to a relay or switchwithin safety system 54. Safety system 54 can be integral with orseparate from transmitter 50. Further, in embodiments where transmitter50 is separate from safety system 54, transmitter 50 can be physicallyremote from system 54.

Safety system 54 is operably coupled to switch 56 to selectively coupletransmitter 50 to heater power source 29. Additionally, safety system 54is also operably coupled to pneumatic valve 58 to selectively couplesource 60 to transmitter 50. Accordingly, once flame scanner reports, orotherwise indicates a flameout condition, safety system 50 can operatevalve 58 to direct reference or purge gas between the measurement celland the diffuser. In fact it is preferred that valve 58 have anon-energized state that is normally open such that safety system 54must energize valve 58 to cease calibration gas flow. While it ispreferred that air or calibration gas be used, any suitablenon-flammable purge gas can be used. Moreover, a pump can be employed todirect any suitable purge gas, including air, into the calibration gasport. Those skilled in the art will appreciate that safety system 54resembles existing automatic calibration modules, and in fact somemodule may be able to be programmed, or otherwise configured, such thata flameout signal will automatically engage calibration gas. Furtherstill, those skilled in the art will recognize that the various valvesand signal processing circuits of safety system 54 could be embodiedwholly within an in situ process gas analyzer in accordance withembodiments of the present invention. Thus, in embodiments wheretransmitter 50 includes a suitable valve coupled to the calibration gasinlet, transmitter 50 itself may receive an indication of a flameoutcondition and engage the calibration gas valve to direct calibration gasto the measurement cell to isolate the measurement cell from potentiallycombustible or explosive materials.

It is believed that embodiments of the present invention can bepracticed with legacy in situ combustion transmitters already deployed.Further still, the automatic generation of a flowing gaseous barrierbetween the thermally elevated sensor measurement cell can be useful toprotect the cell from contact with other materials, such as potentiallycorrosive or damaging materials. While embodiments of the presentinvention are believed to provide significant safety and usabilityadvantages, embodiments of the present invention can still be used witha typical flame arrestor.

Embodiments of the present invention generally utilize anelectro-pneumatic system (either disposed within a transmitter orexternally thereto) to introduce purge gas into a calibration port inresponse to an electrical signal that is responsive to burner flamestatus or a suitable manual selection signal. The gas flow purges thevolume in front of the measurement cell and precludes entry ofmaterials, such as flammable gas, to the high-temperature potentialsource of ignition (measurement cell 36). Under normal operation a purgegas valve is energized and precludes calibration gas flow. In the eventthat the safety system, e.g. 54, input is switched, power is removedfrom the valve and the ensuing air flow precludes process gas (fuel)from reaching the heated zone within seconds. This behavior facilitatessafe operation while maintaining the operating temperature, thusproviding a standby, hot startup capability with an emergency safe modewithout removing heater power. It is anticipated that this system canalso be used to optionally de-energize the heater in parallel with thesafety purge. Moreover, it is contemplated that heater control can alsobe specifically selected to allow the measurement cell to cool to atemperature that is suitably below a flashpoint of the process gas. Oncesuch temperature is confirmed, the reference and/or calibration gas flowcould be reduced. Thus, in such a hybrid embodiment, not only is themeasurement cell at a temperature that is below the flashpoint but someflowing gaseous barrier is also employed.

FIG. 4 a is a diagrammatic view of a distal portion of a probe assembly452. Portion 454 includes distal end 456 which houses diffuser 432.During normal operation, as illustrated in FIG. 4 a, process combustiongas diffuses through diffuser 432 and contacts sensor 458. A calibrationgas conduit 460 is coupled to a calibration gas outlet 462 that isdisposed between sensor 458 and diffuser 432. During normal operation,no calibration gas flows through conduit 460 as illustrated at X 464.

FIG. 4 b is a diagrammatic view of probe assembly 452 during acalibration. During calibration, a calibration gas is provided throughconduit 460, which enters probe assembly 432 at port 462. Thecalibration gas flow fills the region between sensor 458 and diffuser432. By virtue of its known constituents, measurements obtained bysensor 458 when calibration gas is proximate sensor 458 allows forerrors to be detected and compensated. Embodiments of the presentinvention generally take advantage of the positioning of outlet 462 toprovide a non-flammable gas such as reference air, calibration gas, orany other suitable gas, when an unsafe condition (such as a flameout ormanual standby condition) is detected.

FIG. 5 is a flow diagram of a method of operating an in situ processcombustion analyzer in accordance with embodiments of the presentinvention. Method 100 begins at block 102 where a heater of the analyzeris engaged to warm a process analytic gas sensor, such as a processanalytic oxygen sensor, to an operating temperature. As set forth above,an operating temperature for typical zirconia oxide oxygen sensors isapproximately 700° Celsius. Method 100 continues at block 104 where theanalyzer waits until the operating temperature has been reached. Oncethe operating temperature has been reached, control passes to block 106where the process analyzer begins providing process gas measurements,such as process oxygen levels. At block 108, a flameout indication isreceived. This flameout indication can be received by an in situ processoxygen analyzer, a device external to the process oxygen analyzer, orboth. Regardless, once the flameout indication is received, controlpasses to block 110 where a source of calibration gas is engaged togenerate a gaseous barrier between the thermally-elevated measurementcell and any materials near the diffuser. Such materials can includeflammable or potentially explosive process gas, but may also include anymaterial that may have deleterious effects on the measurement cell. Thegas flow can be engaged by the transmitter itself and/or an externaldevice upon reception of the flameout indication. While in block 110,the measurement cell of the process gas analyzer is preferablymaintained at an elevated temperature, at least, and preferably at thefull operating temperature. Method 100 remains in the status of block110 until the flameout condition abates, at which time control returnsto block 106 along line 112 and the analyzer begins providingmeasurements. However, in embodiments where the thermal control systemof the probe has allowed the measurement cell to cool to a temperaturethat is below the operating temperature, control returns to block 102,where the heater is energized to heat the measurement cell to theoperating temperature.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A safety system for use with a process gastransmitter, the safety system comprising: an electro-pneumatic valvefluidically interposed between a source of purge gas and a calibrationgas inlet of the process gas transmitter; and an input operably coupledto the electro-pneumatic valve to cause purge gas to flow into thetransmitter during a detected condition.
 2. The safety system of claim2, wherein the electro-pneumatic valve has a normally open state inwhich the purge gas source and the calibration gas inlet on the processgas transmitter are fluidically coupled, and a closed state wherein thepurge gas source and the calibration gas inlet are decoupled.
 3. Thesafety system of claim 1, and further comprising a switch operablycoupled to the input and electrically interposed between a source ofelectrical power and at least one electrical heating element disposedwithin the process gas transmitter.
 4. The safety system of claim 1,wherein the source of purge gas includes a pump that provides the purgegas into the calibration gas inlet of the transmitter.
 5. The safetysystem of claim 1, wherein the input is a flame scanner input.
 6. Thesafety system of claim 1, wherein the input is a standby selectioninput.
 7. A process gas transmitter comprising: a probe assemblyconfigured for exposure to combustion exhaust within a flue, the probeassembly including a diffuser, a measurement cell and a heater disposedproximate the measurement cell to heat the measurement cell to anoperating temperature, wherein the operating temperature is at least ashigh as a flashpoint of fuel of the combustion; an electronics boardoperably coupled to the probe assembly, the electronics board beingconfigured to obtain process gas measurements from the measurement cellwhile the measurement cell is at the operating temperature; a pneumaticvalve fluidically interposed between a source of purge gas and acalibration gas inlet of the probe assembly, the pneumatic valve beingoperably coupleable to an input wherein the pneumatic valve allows flowof purge gas from the source to a location in the probe assembly betweenthe measurement cell and the diffuser when a signal at the inputindicates a condition.
 8. The process gas transmitter of claim 7,wherein the heater is electrically coupled to a source of power andmaintains the measurement cell at the operating temperature during thecondition.
 9. The process gas transmitter of claim 7, wherein the sourceof purge gas includes a pump operably to direct purge gas to thelocation in the probe assembly between the measurement cell and thediffuser.
 10. The process gas transmitter of claim 7, wherein the inputis a flame scanner input and the condition is a flameout condition. 11.The process gas transmitter of claim 7, wherein the input is a standbyselection input and the condition is a selected standby condition.